A grain-oriented electrical steel sheet used as the magnetic iron core for various electric apparatuses generally contains 2 to 7% Si and has a product crystal structure highly accumulated to {110}<001> orientations. The product quality of a grain-oriented electrical steel sheet is evaluated by both iron loss properties and excitation properties. Reduction of iron loss is as a result of reduction of energy loss taken away as thermal energy when a grain-oriented electrical steel sheet is used in an electric apparatus and therefore is desirable from the viewpoint of energy saving.
Meanwhile, the improvement of excitation properties makes it possible to increase the designed magnetic flux density of an electric apparatus and therefore is desirable from the point of view of reducing the size of the apparatus. Since the accumulation of a product crystal structure to {110}<001> orientations is desirable in order to improve the excitation properties and also reduce iron loss, various research has been carried out and various production technologies developed recently.
One of the typical technologies for the improvement of magnetic flux density is the production method disclosed in Japanese Examined Patent Publication No. S40-15644. This is a production method wherein AlN and MnS function as inhibitors and a high reduction ratio exceeding 80% is employed at the final cold rolling process. By this method, a grain-oriented electrical steel sheet having crystal grains accumulated to {110}<001> orientations and having a high magnetic flux density of 1.870 T or more in terms of B8 (a magnetic flux density at 800 A/m) can be obtained.
However, a magnetic flux density B8 obtained by the method is about 1.88 to at most 1.95 T and the value is only about 95% of the saturation magnetic flux density 2.03 T of a 3% silicon steel. Nevertheless, in recent years, the social demand for energy saving and conservation of resources has been growing increasingly severe and the demand for the reduction of iron loss and the improvement of the magnetization properties of a grain-oriented electrical steel sheet has also been increasing. Therefore, further improvement of magnetic flux density is in strong demand.
As a technology for improving magnetic flux density, Japanese Examined Patent Publication No. S58-50295 proposes the temperature gradient annealing method. By this method, a product having not less than 1.95 T in B8 was produced stably for the first time. However, when the method is applied to a coil having a weight on an industrial scale, the method requires heating an end face of the coil and cooling the other end face thereof to create a temperature gradient and causes large thermal energy loss. Therefore, there has been a problem in the application of the method to industrial production.
In this connection, as a technology to improve magnetic flux density, the method wherein Bi of 100 to 500 g/t is added to molten steel is disclosed in Japanese Unexamined Patent Publication No. H6-88171 and a product having B8 of 1.95 T or more has been produced. Further, the method wherein Bi is contained from 0.0005 to 0.05% as a constituent component in a base material and the material is rapidly heated to a temperature range of 700° C. or higher at a heating rate of 100° C./sec. or more before decarburization annealing is disclosed in Japanese Unexamined Patent Publication No. H8-188824, and by this method, it is possible to stabilize secondary recrystallization over the length and width of a coil and to stably obtain B8 of 1.95 T or more at any point in the coil industrially.
It is believed, as disclosed in Japanese Unexamined Patent Publication No. H6-207216 and others, that Bi accelerates the precipitation of fine MnS and AlN functioning as inhibitors, thus raises inhibitor strength, and is advantageous to the selective growth of the crystal grains having little deviation from the ideal {110}<001> orientations.
In particular, it is well known that the precipitation control of AlN functioning as an inhibitor greatly depends on the temperature of hot band annealing or annealing prior to the finish cold-rolling process among a plurality of cold-rolling processes incorporating intermediate annealing in between, and therefore optimization of the temperature has been adopted.
The following methods are employed in the case of a base material containing Bi: the method wherein hot band annealing or annealing prior to the finish cold-rolling process among a plurality of cold-rolling processes incorporating intermediate annealing in between is applied for 30 sec. to 30 min. in a temperature range from 850° C. to 1,100° C. as disclosed in Japanese Unexamined Patent Publication No. H6-212265; the method wherein the temperature of annealing prior to finish cold rolling is controlled in accordance with the excessive amount of Al in steel as disclosed in Japanese Unexamined Patent Publication No. H8-253815; and the method wherein an average cooling rate of a hot band is controlled and a temperature of annealing prior to finish cold rolling is controlled in the range from 2,400×Bi (wt %)+875° C. to 2,400×Bi (wt %)+1,025° C. in accordance with a Bi content as disclosed in Japanese Unexamined Patent Publication No. H11-124627. A feature of all of these methods is that the appropriate temperature range of annealing prior to finish cold rolling is lower than that in the case of not adding Bi.
However, since equipment for annealing prior to finish cold rolling is generally not designed so as to exclusively process Bi contained materials, it has been necessary to change the temperature from a higher temperature for a material not containing Bi when a Bi contained material is processed at a lower temperature, and poor secondary recrystallization or, even when secondary recrystallization occurs, poor magnetic property in terms of low magnetic flux density has sometimes arisen at the temperature change portion. Furthermore, a coil for temperature adjustment is sometimes used in the event of temperature change, but this measure is not desirable, since it reduces productivity.
In the meantime, as methods for reducing iron loss, various methods of magnetic domains refinement are disclosed including: the method wherein laser treatment is applied to a steel sheet disclosed in Japanese Examined Patent Publication No. S57-2252; the method wherein mechanical strain is introduced to a steel sheet disclosed in Japanese Examined Patent Publication No. S58-2569; and other methods. In general, the iron loss of a grain-oriented electrical steel sheet is evaluated by W17/50 (energy loss under the excitation conditions of 1.7 T in B8 and 50 Hz) stipulated in JIS C2553 and classified. In recent years, cases where an excitation magnetic flux density is raised to 1.7 T or more in an attempt to downsize a transformer and, even when a magnetic flux density is designed to be 1.7 T, a local magnetic flux density of a transformer iron core is raised to 1.7 T or more, and a steel sheet having a reduced iron loss at a high magnetic flux density (W19/50 for example) is desired.
With regard to a grain-oriented electrical steel sheet having a reduced iron loss in a high magnetic flux density, Japanese Unexamined Patent Publication No. 2000-345306 discloses the method wherein the deviation of the crystal orientations of a steel sheet from the ideal {110}<001> orientation is controlled to not more than five degrees on average and the average magnetic domain width of the steel sheet at 180° C. is controlled in the range from over 0.26 to 0.30 mm, or the area percentage of magnetic domains having a magnetic domain width of over 0.4 mm in the steel sheet is controlled in the range from over 3 to 20%. As a method for producing such a grain-oriented electrical steel sheet, Japanese Unexamined Patent Publication No. 2000-345305 discloses the method wherein a steel sheet is heated to 800° C. or higher at a heating rate of 100° C./sec. or more immediately before decarburization annealing. However, the high magnetic field iron loss of a steel sheet produced by the method is 1.13 W/kg in W19/50 at the lowest, and thus grain-oriented electrical steel sheet having still lower iron loss at a high magnetic flux density is desired.
In the case where Bi is contained in a base material, as disclosed in Japanese Unexamined Patent Publication Nos. H6-89805 and 2000-26942, the crystal grains of a product coarsen, therefore the magnetic domain width increases, conventional measures for magnetic domains refinement are not sufficient to narrow the magnetic domain width, and consequently there has been room for further decreasing iron loss at high magnetic flux density.
Further, as disclosed in many patent publications, when Bi is contained in a steel, a glass film that functions as an insulating film has not been formed stably in the width direction.
Moreover, as a technology for rapidly heating a steel sheet immediately before decarburization annealing, Japanese Unexamined Patent Publication No. H11-61356 discloses the technology for producing a grain-oriented electrical steel sheet excellent in film adhesiveness and magnetic properties through the processes of: carrying out the heating process in decarburization annealing in a rapid-heating chamber installed next to a decarburization annealing furnace; controlling the ratio PH2O/PH2 in the rapid-heating chamber in the range from 0.65 to 3.0; rapidly heating the strip to a temperature of 800° C. or higher at a heating rate of 100° C./sec. or more; controlling the resident time in the temperature range of 750° C. or higher in the rapid-heating chamber to 5 sec. or less; and further processing the strip by controlling the ratio PH20/PH2 in the decarburization annealing furnace in the range from 0.25 to 0.6. Further, Japanese Unexamined Patent Publication No. 2000-204450 discloses the method for producing a grain-oriented electrical steel sheet excellent in film adhesiveness and magnetic properties by heating a steel sheet to 800° C. or higher at a heating rate of 100° C./sec. or more and controlling an oxygen partial pressure and a vapor partial pressure in an atmosphere in the temperature range. However, even by those methods, when Bi is contained in a steel, it is impossible to form a primary film uniformly in a coil.
Further, Japanese Unexamined Patent Publication No. H8-188824 discloses the technology for obtaining a high magnetic flux density uniformly in a coil by: containing 0.0005 to 0.05% Bi in a base material; heating the coil to a temperature range of 700° C. or higher at a heating rate of 100° C./sec. or more in an atmosphere having a ratio PH20/PH2 of 0.4 or less before applying decarburization annealing; thus controlling the amount of SiO2; and stabilizing the behavior of absorbing and disgorging nitrogen in finish annealing. Such heat treatment is applied generally by using an electrical device for induction heating or conduction heating, and therefore it is commonly used to control an H2 concentration to 4% or less from the viewpoint of explosion-protection. Therefore, in order to secure an atmosphere wherein the ratio PH20/PH2 is controlled to 0.4 or less, it is necessary to stabilize operation at a low dew point, and thus a dehumidifier or the like is required, which results in increased equipment cost. In addition, a problem thereof is that the dew point must be controlled so as to deal with the least variation of a hydrogen concentration and therefore flexibility of operation is greatly hampered.
Next, an electrically insulative film formed on the surface of a grain-oriented electrical steel sheet is explained. Such a film plays a role not only of maintaining insulation, but also of imposing a tensile stress on a steel sheet and reducing iron loss by making use of the fact that the coefficient of thermal expansion of the film is lower than that of the steel sheet. Further, a good insulating film is important also in a transformer manufacturing process. In particular, in the case of a wound-core type transformer, bend forming is applied to a grain-oriented electrical steel sheet and therefore a film may sometimes exfoliate. For this reason, a film is also required to have excellent film adhesiveness.
Such an insulating film of a grain-oriented electrical steel sheet is composed of two films; a primary film and a secondary film. A primary film is formed by making SiO2 that is formed on a steel sheet surface in decarburization annealing react to an annealing separator that is applied thereafter in the finish annealing process. In general, an annealing separator is component mainly of MgO and reacts to SiO2 and forms Mg2SiO1. Finish annealing is generally applied to a steel sheet in the state of a coil and is influenced by temperature deviation in the coil and the distributability of an atmosphere between steel sheet layers. Therefore, a challenge is to form a primary film uniformly, and various methods have tried to solve the problem with regard to a decarburization annealing process, MgO functioning as an annealing separator, finish annealing process conditions and others.
As methods for optimizing an oxide layer formed on the surface of a steel sheet subjected to decarburization annealing, Japanese Unexamined Patent Publication No. H11-323438 discloses the method wherein PH20/PH2 in a soaking zone is kept lower than PH20/PH2 in a heating zone, Japanese Unexamined Patent Publication No. 2000-96149 the method wherein a heating rate is controlled to 12 to 40° C./sec. on average in a temperature range from ordinary temperature to 750° C. and to 0.5 to 10° C./sec. on average in a temperature range from 750° C. to a soaking temperature, and Japanese Unexamined Patent Publication No. H10-152725 the method wherein the an oxygen amount on the surface of a steel sheet after decarburization annealing is controlled in the range from 550 to 850 ppm.
Further, with regard to an annealing separator composed mainly of MgO and applied after decarburization annealing, Japanese Unexamined Patent Publication No. H8-253819 discloses the method wherein the coating amount of an annealing separator is controlled to 5 g/m2 or more, and Japanese Unexamined Patent Publication No. H10-25516 the method wherein an Ig-loss value is controlled in the range from 0.4 to 1.5%.
Furthermore, with regard to a Ti chemical compound, represented by TiO2, used as an additive to MgO, many technologies have been proposed. As such methods in the case of a base material not containing Bi, Japanese Examined Patent Publication No. S49-29409 disclosed the method wherein anatase-type TiO2 of 2-20 is blended with MgO of 100 as parts by weight, Japanese Examined Patent Publication No. S51-12451 the method wherein a Ti chemical compound of 2-40 is blended with an MgO chemical compound of 100 as parts by weight, Japanese Unexamined Patent Publication No. S54-128928 the method wherein TiO2 of 1-10 as parts by weight and SiO2 of 1-10 as parts by weight are contained as parts by weight, and Japanese Unexamined Patent Publication No. H5-195072 the method wherein a Ti chemical compound of 1-40 in terms of TiO2 is blended as parts by weight and an atmosphere containing nitrogen is used at the first stage of purification annealing.
As such methods in the case of a base material containing Bi, Japanese Unexamined Patent Publication No. 2000-96149 discloses the method wherein SnO2, Fe2O3, Fe3O4 and MoO2 are added by 0-15 as parts by weight, further TiO2 is added by 1.0-15 as parts by weight, and by so doing, film adhesiveness is improved. However, since a finish annealing process is generally applied to a steel sheet in the state of a coil, temperature deviation and the deviation of the distributability of an atmosphere occur in the coil, and therefore it has been difficult to control dissociative reaction of such SnO2, Fe2O3, Fe3O4 and MoO3. Further, Japanese Unexamined Patent Publication No. 2000-144250 discloses the method wherein a Ti chemical compound of 1-40 is blended as parts by weight, the nitrogen concentration is raised temporarily in accordance with the amount of the Ti chemical compound after the completion of secondary recrystallization, and by so doing, Ti is prevented from intruding into a steel. However, a problem of the method has been that the time of completion of secondary recrystallization is difficult to judge because of the temperature deviation in a coil as stated above.
With regard to a finish annealing process, Japanese Unexamined Patent Publication No. H9-3541 discloses the technology wherein the flow rate of an atmosphere gas at finish annealing is controlled so that the value of “atmosphere gas flow rate/(furnace inner volume−steel sheet volume)” may be not less than 0.5 Nm3/hr./m3. However, by the technology, the distributability of an atmosphere deviates between steel sheet layers in a coil, and therefore a desired effect is not obtained.
As explained above, in the case of a steel containing Bi, it is difficult to form a primary film uniformly by the aforementioned methods. Moreover, adhesiveness deteriorates when an insulating film having a film tension is applied, and poor secondary recrystallization, poor magnetic property in terms of low magnetic flux density occurs in the longitudinal direction when annealing is applied to a steel sheet in the state of a coil. Therefore, a problem of the above methods has been that it is difficult to obtain reduced iron loss at high magnetic flux density and good film adhesiveness distributing uniformly in the width and longitudinal directions when an insulating film is applied after finish annealing.